OBJECTIVE: To determine the role of endogenous hormones in male parental care. RESULTS Inexperienced and experienced father cotton-top tamarins show different patterns of prolactin release and possibly communication between mates. Testosterone levels appear to have an inverse relationship with prolactin during the 15 days before and following parturition. Female elevated prolactin levels fall within two weeks after infants die whereas with nursing infants prolactin levels continue high. We are continuing to study the role of hormones in parental care in the cotton-top tamarin. Six experienced fathers and five first-time fathers are being examined for their hormonal changes occurring during the pregnancy of their mate. Preliminary information has shown that all experienced fathers tested (4) have a rise in prolactin levels during the second and third month while all of the first-time fathers do not show an elevation of prolactin until the fifth and sixth month of gestation. These differences in prolactin levels may indicate a difference between experienced and inexperienced males in their perception of pregnancy cues from the female. We are investigating hormonal changes in males and females during the gestational period as well as behavioral responses between the male and female during this time to understand the communication that occurs between mates during pregnancy. Urine from ten male cotton-top tamarin males have been analyzed for the levels of prolactin, testosterone, DHT and cortisol for the 15 days prior to and the 15 days following the birth of infants to their mate. Preliminary information on 4 males showed that during this time there was an inverse relationship between the levels of testosterone and prolactin. We are continuing to examine this relationship and its significance to parenting behaviors in this species. Female tamarins show elevated levels of prolactin throughout the six weeks following birth of infants unless the infants die; causing prolactin levels to decline to baseline levels within two weeks. FUTURE DIRECTIONS We plan to examine the influence of parenting behaviors on hormonal changes in male tamarins. KEY WORDS parental care, behaviors, prolactin, testosterone, New World primates